1. Technical Field
The present invention relates to a numerical control ("NC") system for a five-axis precision positioning and support of a machine tool with respect to a workpiece surface in a work envelope.
2. Background Art
The precision machining of large workpieces requires the use of a wide array of expensive machine tools such as full size models and gauges, templates, fixtures, and drill-sets. These tools have a substantial acquisition and maintenance costs, as well as costs related to their storage, property management, inspection, reinspection, and accountability. In addition, the manufacturing tolerances and repeatability achievable with these tools is limited.
For example in the aerospace industry, large airframe components such as fuselage sections can be precision machined only with the use of very costly full size models and gauges. A typical series of models needed to drill precision holes is shown in FIGS. 1A-1B. As shown in FIG. 1A, the first step in this process is to fabricate a male master model 100 of a fuselage section, which model is made of metal or plaster and has projections 105 of the size and at the locations required for the holes to be drilled in the fuselage section. A female plaster cast 110 is formed over the model 100, which cast has apertures 115 formed over the projections 105. As shown in FIG. 1B, a male cast back 120 is formed from the plaster cast 110, which cast back is also made from plaster. Again, projections 125 are formed by the plaster flowing into the apertures 115 in the cast 110. Finally, a drill bonnet 130 made of a composite material, such as fiberglass or graphite composite, is formed over the cast back 120. The bonnet 130 has apertures 135 of the correct size and at the correct locations where holes are required to be drilled.
The first step in using the bonnet 130 is to fasten a fuselage section into an assembly jig using bracing means, or "details", and locator pins to provide a reference position for the fuselage. The bonnet 130 is then secured adjacent the fuselage section and aligned with the section using the locator pins. The bonnet 130 then serves as a drilling template through which holes are drilled into the fuselage section.
The cost to fabricate a typical drill bonnet 130 can average $1 million and take from one to 12 weeks. For the F-18 aircraft, 900 bonnets are needed to drill all the fuselage holes. Thus, the total cost for the drill bonnet tool family for the F-18 is approximately $1 billion. Full scale interior models, called master gages, are also required to precisely locate and drill holes in details which are attached to interior structures of the assembly jig. These details are used to locate the bulkheads, frames and ribs of the aircraft. Such master gages can cost between $5-10 million each and the F-18 requires 33 such master gages, for a total master gage tool family cost of approximately $250 million.
One object of the invention is to eliminate the need for these costly tool families and replace them with a machine tool locating system made from standardized parts to reduce cost and fabrication time. Another object of the invention is to improve the accuracy of hole location by eliminating the cumulative tolerance resulting from the use of multiple master models and gages, and related molds.
Another object of the invention is to increase the speed with which an assembly jig can be prepared to machine a new workpiece, or implement engineering changes to an existing workpiece design. Previously, new master models and gages would have to be fabricated for either a new aircraft component or changes to an existing one, requiring from four to 24 weeks to prepare. A positioning system of invention can locate machine tools directly from machine design software, reducing this aircraft change time to one or two days.